1. Field of the Invention:
This invention relates to leadframes for integrated circuit (IC) packages, and more particularly, to a leadframe which is plated with silver (Ag) and palladium (Pd) in distinctive manners so as to provide suitable bondability, molding compound characteristic, and solderability for the packaging of a chip in the IC package. This invention also relates to a method for manufacturing the leadframe.
2. Description of Related Art:
In the manufacture of semiconductor devices on integrated circuits (IC), the final step is to mount the chip on a leadframe and then enclose the chip and part of the leadframe in a plastic casing to form the so-called IC package. The IC package can be easily mounted on a printed circuit board for interconnection of the electronic devices on the chip with external circuitry. A leadframe should be manufactured in such a manner as to provide good bondability, molding compound characteristic, and solderability, so that it can facilitate the packaging process. To provide these characteristics, various coatings are formed on the leadframe.
A conventional method for providing suitable bondability for the interconnection between gold wires and leads of a leadframe is to plate silver (Ag) on the bonding area of the leadframe. After the chip is sealed in a plastic casing, the external leads of the IC package are plated with a layer of the alloy of tin/lead (Sn/Pb) so as to provide suitable solderability for the external leads of the IC package to be easily mounted on a printed circuit board by soldering.
One drawback to the process of plating the Sn/Pb alloy on the external leads, however, is that the high-temperature condition of the plating process can cause damage to the IC package and the chip. Moreover, the use of the tin/lead (Sn/Pb) alloy can cause the problem of heavy metal pollution to the environment.
Further, since the plating of silver (Ag) on the bonding area is carried out prior to the packaging of the chip in a plastic casing and the plating of the tin/lead (Sn/Pb) alloy on the external leads is carried out after the chip is packaged, the automation of the overall packaging process is difficult to implement. To solve this problem, the so-called pre-plated frame (PPF) technology is developed.
The PPF technology involves the plating of palladium (Pd) instead of silver (Ag) and tin/lead (Sn/Pb) on the leadframe. Conventional PPF methods include U.S. application Ser. No. 874,916 filed on Jun. 16, 1986, and U.S. application Ser. No. 174,060 filed on Mar. 28, 1988 by Texas Instruments (TI) Incorporated, to name a few. Since the palladium (Pd) metal is good both in bondability and solderability, it can be plated over the entire leadframe prior to the packaging process so that the ensuring packaging process can be carried out easily by automation. In this case, the plating of the Sn/Pb alloy in the prior art is eliminated. Therefore, the PPF technology is quite an environmentally friendly solution.
By the PPF technology, however, in order to allow the leadframe to be erosion-resistant, a film of nickel (Ni) and a film of the alloy of palladium/nickel (Pd/Ni) are plated on the leadframe before plating the palladium (Pd) metal. Since palladium (Pd) is a precious metal which is quite costly to use, the platings are usually very thin, typically from 0.075 .mu.m to 0.1 .mu.m (micrometer) only. Due to such a small thickness, the thin palladium (Pd) platings can be easily ruined during the packaging process, which causes the forming of undesired pin holes in the platings. The forming of these pin holes will expose the nickel (Ni) and palladium/nickel (Pd/Ni) platings, so that it will significantly degrade the bondability and solderability of the leadframe.